Some conventional four-wheel-drive vehicles include a drive power distribution device for distributing drive power generated by a drive power source, such as an engine, to main driving wheels and auxiliary driving wheels. In this kind of four-wheel-drive vehicles, when the front wheels are main driving wheels while the rear wheels are auxiliary driving wheels, for example, the drive power generated at the drive power source is transmitted to the front wheels through a front drive shaft and a front differential while being transmitted to a drive power distribution device including a multiple disc clutch through a propeller shaft. Hydraulic fluid is then supplied at a predetermined pressure from a hydraulic control device to the drive power distribution device to control the engagement pressure of the drive power distribution device. A predetermined proportion of the drive power from the drive power source is thereby transmitted to the rear wheels.
Examples of the hydraulic control device configured to control the hydraulic pressure supplied to the multiple disc clutch of the drive power distribution device include hydraulic control devices shown in Patent Literatures 1 and 2. Each of the hydraulic control devices shown in Patent Literatures 1 and 2 includes an electric oil pump supplying hydraulic fluid to a hydraulic chamber that presses the multiple disc clutch. The electric oil pump and hydraulic chamber are connected with a hydraulic pressure supply path. The hydraulic control device controls the revolutions of the electric pump so that the discharge value of the electric pump is equal to the required hydraulic pressure for the hydraulic clutch. The hydraulic control device described in Patent Literature 2 controls motor drive of the electric pump so as to generate hydraulic pressure according to the distribution ratio of drive power. The hydraulic control devices of Patent Literatures 1 and 2 are configured to supply hydraulic pressure necessary for the hydraulic clutch by driving the electric pump. The electric oil pump therefore needs to be always operated while the hydraulic clutch is engaged. Accordingly, it is difficult to guarantee the durability of the motor (brush wear) when the motor to drive the electric oil pump is a brushed motor.
In this light, Patent Literature 3 proposes a hydraulic pressure sealed-type hydraulic control device using a motor and a solenoid valve. In this hydraulic pressure sealed-type hydraulic control device, the hydraulic pressure path to supply hydraulic fluid from an oil pump driven by the motor to a piston chamber of a clutch for distributing drive power is provided with a hydraulic fluid sealing valve to seal hydraulic fluid and a solenoid valve (an on-off valve) to open and close the fluid path between the hydraulic fluid sealing valve and piston chamber. In order to pressurize the piston chamber, the hydraulic pressure sealed-type hydraulic control device closes the solenoid valve and drives the oil pump with the motor in a stepwise manner to perform control so that the pressure in the piston chamber become commanded hydraulic pressure. In order to depressurize the piston chamber, the hydraulic pressure sealed-type hydraulic control device disables drive of the oil pump and opens and closes the solenoid valve in a stepwise manner to perform control so that the pressure of the piston chamber becomes the commanded hydraulic pressure. In such a manner, the motor is driven only to pressurize the piston chamber and is not driven to depressurize the piston chamber. This can reduce the frequency of use of the motor, improving the durability.